Semiconductor device and method of manufacturing the same

ABSTRACT

According to an aspect of the invention, there is provided a semiconductor device including a plurality of memory cells, comprising a plurality of floating gate electrodes which are formed on a tunnel insulating film formed on a semiconductor substrate and have an upper portion which is narrower in a channel width direction than a lower portion, an interelectrode insulating film formed on the floating gate electrodes, and a control gate electrode which is formed on the interelectrode insulating film formed on the floating gate electrodes and partially buried between the floating gate electrodes opposing each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 13/114,590, filedMay 24, 2011 (now U.S. Pat. No. 8,076,711), which is a continuation ofU.S. application Ser. No. 12/717,408, filed Mar. 4, 2010 (now U.S. Pat.No. 7,960,230), which is a divisional of U.S. application Ser. No.11/226,287, filed Sep. 15, 2005 (now U.S. Pat. No. 7,705,391), which isbased upon and claims the benefit of priority from prior Japanese PatentApplication No. 2004-275528, filed Sep. 22, 2004. The contents of theseapplications are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having afloating gate electrode and a method of manufacturing the same.

2. Description of the Related Art

FIG. 9A is a view showing the structure of conventional nonvolatilememory cells. FIG. 9A is a sectional view in the direction of word lines(channel width direction: a direction perpendicular to a direction inwhich a channel current flows). A tunnel insulating film 2 is formed ona silicon substrate 1. A plurality of floating gate electrodes 3 arearranged on the tunnel insulating films 2 while being separated fromeach other by a predetermined distance. An element isolation insulatingfilm 4 is buried between the lower portions of the floating gateelectrodes 3. An interelectrode insulating film 5 is formed on part ofthe side surfaces and the upper surface of each floating gate electrode3, and the upper surface of the element isolation insulating film 4. Acontrol gate electrode 6 is formed on the interelectrode insulating film5.

FIG. 9B is a view showing the structure of the nonvolatile memory cells.FIG. 9B is a sectional view in the direction of bit lines (channellength direction: a direction in which a channel current flows). Aplurality of cell diffusion layers 7 are formed in the surface of thesilicon substrate 1. A plurality of stacking cells CE each including thefloating gate electrode 3, interelectrode insulating film 5, and controlgate electrode 6 are arranged on the tunnel insulating film 2 betweenthe cell diffusion layer 7 while being separated from each other by apredetermined distance. An interlayer dielectric film 8 is buriedbetween the stacking cells CE.

As shown in FIG. 9B, the floating gate electrodes 3 adjacent in the bitline direction (channel length direction) oppose each other via theinterlayer dielectric film 8. Along with advanced micropatterning ofmemory cells, the opposing distance shortens. Hence, a parasiticcapacitance C between the opposing surfaces of the adjacent floatinggate electrodes 3 increases. For this reason, the write/erase state ofan adjacent cell affects the operation characteristic of a cell ofinterest, and so-called intercell interference occurs, resulting in anoperation error in the memory.

As shown in FIG. 9A, the width of the control gate electrode buriedbetween the floating gate electrodes 3 adjacent in the word linedirection (channel width direction) becomes small as micropatterning ofmemory cells progresses. Consequently, as the buried portion of thecontrol gate electrode is normally made of a semiconductor containing adopant impurity, the dopant impurity concentration becomes low in theburied portion. Hence, depletion occurs in the buried portion when ahigh electric field is applied in a write/erase operation. For thisreason, the decrease in electric capacitance between the control gateelectrode 6 and the floating gate electrode 3 is not negligible, andoperation errors in memory cells occur. When depletion occurs in theburied portion, the electric shielding effect between the floating gateelectrodes 3 on both sides of the buried portion decreases. For thisreason, the probability of occurrence of memory operation errors causedby the intercell interference also becomes high.

Jpn. Pat. Appln. KOKAI Publication No. 8-88285 discloses an EEPROM. Inthis EEPROM, element formation regions each having a convex shape and around upper end are formed on a substrate surface while being isolatedfrom each other by isolation trenches. A tunnel film, floating gateelectrode, capacitance insulating film, and control gate electrode areformed on the element formation region.

Jpn. Pat. Appln. KOKAI Publication No. 11-177066 discloses an EEPROMmanufacturing method. In this method, isolation trenches are formed byself-aligning with floating sate electrodes. After the entire surface isoxidized, the isolation trenches are filled with an insulating film. Thesurface of the insulating film is removed to expose the side surface ofthe floating gate electrodes. A second interelectrode insulating film isformed on the entire surface, and control gate electrodes are formed.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided asemiconductor device including a plurality of memory cells, comprising:a plurality of floating gate electrodes which are formed on a tunnelinsulating film formed on a semiconductor substrate and have an upperportion which is narrower in a channel width direction than a lowerportion; an interelectrode insulating film formed on the floating gateelectrodes; and a control gate electrode which is formed on theinterelectrode insulating film formed on the floating gate electrodesand partially buried between the floating gate electrodes opposing eachother.

According to another aspect of the invention, there is provided a methodof manufacturing a semiconductor device, comprising: forming a pluralityof floating gate electrodes, which have an upper portion which isnarrower in a channel width direction than a lower portion, on a tunnelinsulating film formed on a semiconductor substrate; filling a gapbetween side surfaces of the floating gate electrodes opposing eachother with an element isolation insulating film; removing part of theelement isolation insulating film to partially expose the side surfacesof the floating gate electrodes; forming an interelectrode insulatingfilm on the floating gate electrodes and forming part of theinterelectrode insulating film on the exposed side surfaces of thefloating gate electrodes; and forming a control gate electrode on thefloating gate electrodes, the control gate electrode being partiallyburied between the floating gate electrodes opposing each other.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIGS. 1A and 1B are views showing the structure of a nonvolatile memoryaccording to the first embodiment;

FIG. 2 is a view showing the structure of nonvolatile memory cellsaccording to the first embodiment;

FIG. 3 is a sectional view showing a modification of the nonvolatilememory cells according to the first embodiment;

FIG. 4 is a sectional view showing another modification of thenonvolatile memory cells according to the first embodiment;

FIGS. 5A to 5D are views showing the procedures of manufacturing thenonvolatile memory cell according to the first embodiment;

FIGS. 6A and 6B are views showing the procedures of manufacturing thenonvolatile memory cell according to the first embodiment;

FIGS. 7A to 7C are views showing the procedures of manufacturingnonvolatile memory cell according to the second embodiment;

FIGS. 8A to 8D are views showing the procedures of manufacturingnonvolatile memory cell according to the third embodiment; and

FIGS. 9A and 9B are views showing the structure of conventionalnonvolatile memory cells.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A and 1B are views showing the structure of a nonvolatile memory(NAND flash memory) as a semiconductor device according to the firstembodiment of the present invention. FIG. 1A is a plan view of memorycells of the NAND flash memory. FIG. 1B is an equivalent circuit diagramof the memory cells.

Referring to FIGS. 1A and 1B, reference symbols M1 to M8 denotenonvolatile memory cell portions; S1 and S2, select transistor portions;CG1 to CG8, control gates (word lines); SG1 and SG2, select gates; BL1and BL2, bit lines; and Vss, a source voltage.

FIG. 2 is a view showing the structure of nonvolatile memory cellsaccording to the first embodiment of the present invention. FIG. 2 is asectional view in the word line direction (channel width direction).

A tunnel insulating film 2 is formed on a silicon substrate 1. Aplurality of floating gate electrodes 3 are arranged on the tunnelinsulating films 2 while being separated from each other by apredetermined distance. The width of the upper portion of each floatinggate electrode 3 is smaller than that of the lower portion. An elementisolation insulating film 4 is buried between the adjacent floating gateelectrodes 3 up to a position in the height direction in the regionwhere the floating gate electrodes 3 are wider at their lower portions.An interelectrode insulating film 5 is formed on the floating gateelectrodes 3 and element isolation insulating film 4. A control gateelectrode 6 is formed on the interelectrode insulating film 5. Thecontrol gate electrode 6 is partially buried between the adjacentfloating gate electrodes 3.

In this memory cell structure, the area of the side surface of thefloating gate electrode 3 perpendicular to the bit line direction(channel length direction) can be reduced while ensuring the opposingarea (the area of the interelectrode insulating film 5) between thefloating gate electrode 3 and the control gate electrode 6, unlike theconventional structure shown in FIG. 9A. For this reason, the parasiticcapacitance between the floating gate electrodes 3 adjacent in the bitline direction (channel length direction) can be reduced whilesuppressing the increase in cell operating voltage by ensuring thecoupling ratio of the memory cells. Accordingly, the frequency ofoccurrence of memory cell operation errors can be reduced.

In this memory cell structure, a width W1 of the upper portion of theburied portion of the control gate electrode 6 is larger than a widthobtained by subtracting a value twice a thickness W3 of theinterelectrode insulating film 5 from a minimum interval W2 between theopposing floating gate electrodes 3. When the buried portion of thecontrol gate electrode 6 is made of a semiconductor containing a dopantimpurity, the dopant can sufficiently be diffused to the bottom of thecontrol gate electrode 6 and consequently the dopant impurityconcentration becomes sufficiently high in the buried portion. As aresult, depletion hardly occurs in the buried portion when a highelectric field is applied in a write/erase operation. For this reason,any operation error in memory cells which is caused by the decrease inelectric capacitance between the control gate electrode 6 and thefloating gate electrode 3 can be avoided. In addition, any memoryoperation error caused by the decrease in electric shielding effectbetween the adjacent floating gate electrodes 3 can also be avoided.

FIG. 3 is a sectional view showing a modification of the nonvolatilememory cells according to the first embodiment. In this modification,the element isolation insulating film 4 is buried between the adjacentfloating gate electrodes 3 up to a position in the height direction inthe region where the floating gate electrodes 3 become narrow. In thismemory cell structure, the bottom width of the control gate electrode 6becomes sufficiently wide, and consequently depletion more hardly occursin the above-described buried portion of the control gate electrode 6.For this reason, the ratio of memory operation errors caused bydepletion can further be reduced.

The position in the height direction in which the width of the floatinggate electrode 3 decreases can be set at any portion in FIGS. 2 and 3.The position is preferably set as closer as possible to the tunnelinsulating film 2 because a large effect can be obtained. The width ofthe floating gate electrode 3 need not always change at one step asshown in FIGS. 2 and 3. It may change at two steps, as shown in FIG. 4,or at three or more steps.

FIGS. 5A to 5D and 6A and 6B are views showing the procedures ofmanufacturing the nonvolatile memory cell according to the firstembodiment. The procedures of manufacturing the memory cell shown inFIG. 2 will be described with reference to FIGS. 5A to 5D and 6A and 6B.FIGS. 5A to 5D are sectional views of the nonvolatile memory cell in theword line direction (channel width direction). FIGS. 6A and 6B showsectional views of the nonvolatile memory cell in the bit line direction(channel length direction) on the left side and sectional views of thenonvolatile memory cell in the word line direction (channel widthdirection) on the right side.

First, as shown in FIG. 5A, a desired impurity is doped in a siliconsubstrate 101. A 10-nm thick-tunnel insulating film 102 is formed on thesurface of the silicon substrate 101 by thermal oxidation. After that, aphosphorus-doped polysilicon layer 103 having a thickness of 150 nm isdeposited by low-pressure CVD (Chemical Vapor Deposition) as aprospective floating gate electrode. A stopper film 104 for CMP(Chemical Mechanical Polish) and a mask film 105 for RIE (Reactive IonEtching) are sequentially deposited by low-pressure CVD.

The mask film 105, stopper film 104, and the upper portion of thephosphorus-doped polysilicon layer 103 are sequentially etched by RIEusing a resist mask (not shown). Accordingly, a sidewall portion 201 isformed.

As shown in FIG. 5B, a silicon oxide film is deposited on the entiresurface by low-pressure CVD. Then, non-masking RIE is executed. At thistime, the conditions of non-masking RIE are set such that a sidewallmask film 105 a remains on the sidewall portion 201. As shown in FIG.5C, the exposed region of the polysilicon layer 103 and the tunnelinsulating film 102 are sequentially etched by using the mask film 105and sidewall mask film 105 a as a mask. In addition, the exposed regionof the silicon substrate 101 is etched to form a 150-nm deep elementisolation trench 106. Accordingly, the shape of a floating gateelectrode having a wide lower portion and a narrow upper portion isformed.

As shown in FIG. 5D, a 400-nm thick silicon oxide film 107 a for elementisolation is deposited on the entire surface by plasma CVD to completelyfill the element isolation trench 106. After that, the silicon oxidefilm 107 a and mask film 105 at the surface portion are removed by CMPto planarize the surface. The exposed stopper film 104 is removed bychemical etching. The exposed surface of the silicon oxide film 107 a isremoved by etching using a diluted hydrofluoric acid solution so thatthe silicon oxide film 107 a is etched back to a position in the heightdirection where the floating gate electrode is wide.

As shown in FIG. 6A, a 15-nm thick interelectrode insulating film 109having a three-layered structure including a silicon oxide film/siliconnitride film/silicon oxide film is sequentially deposited on the entiresurface by low-pressure CVD. A 100-nm thick conductive layer 110 havinga two-layered structure including a polysilicon layer/tungsten silicidelayer is sequentially deposited by low-pressure CVD as a prospectivecontrol gate electrode. A mask film 111 for RIE is deposited bylow-pressure CVD. The mask film 111, conductive layer 110,interelectrode insulating film 109, polysilicon layer 103, and tunnelinsulating film 102 are sequentially etched by RIE using a resist mask(not shown) to form slit portions 112 between stacking cells. With thisprocess, the shapes of a floating gate electrode 113 and control gateelectrode 114 are determined.

As shown in FIG. 6B, a 10-nm thick silicon oxide film 115 called anelectrode sidewall oxide film is formed on the exposed surface bycombining thermal oxidation and low-pressure CVD. Cell diffusion layers116 are formed by ion implantation. A BPSG (Boro Phosphor SilicateGlass) film 117 as a prospective interlayer insulating film is formed onthe entire surface by low-pressure CVD. After that, an interconnectionlayer and the like are formed by a known method, thereby completing anonvolatile memory cell.

FIGS. 7A to 7C are views showing the procedures of manufacturing anonvolatile memory cell as a semiconductor device according to thesecond embodiment. The procedures of manufacturing the memory cell shownin FIG. 2 will be described with reference to FIGS. 7A to 7C. FIGS. 7Ato 7C are sectional views in the word line direction (channel widthdirection).

First, as shown in FIG. 7A, a desired impurity is doped in a siliconsubstrate 101. A 10-nm thick tunnel insulating film 102 is formed on thesurface of the silicon substrate 101 by thermal oxidation. After that, aphosphorus-doped polysilicon layer 103 having a thickness of 150 nm isdeposited by low-pressure CVD as a prospective floating gate electrode.The phosphorus-doped polysilicon layer 103 and tunnel insulating film102 are sequentially etched by RIE using a resist mask (not shown). Inaddition, the exposed region of the silicon substrate 101 is etched toform a 150-nm deep element isolation trench.

A 400-nm thick silicon oxide film 107 a for element isolation isdeposited on the entire surface by plasma CVD to completely fill theelement isolation trench. After that, the surface is planarized by CMP.The exposed surface of the silicon oxide film 107 a is removed byetching using a diluted hydrofluoric acid solution to expose the sidesurface of the floating gate electrode 103 by 70 nm.

As shown in FIG. 7B, the exposed surface of a floating gate electrode103 a is removed by 30 nm by isotropic etching using an alkali solution.Accordingly, the floating gate electrode 103 a obtains a wide lowerportion and a narrow upper portion. As shown in FIG. 7C, the exposedsurface of the silicon oxide film 107 a is removed by etching using adiluted hydrofluoric acid solution so that the silicon oxide film 107 ais etched back to a position in the height direction where the floatinggate electrode is wide.

Then, the memory cell structure as shown in FIG. 2 is completed by usingthe method shown in FIGS. 6A and 6B.

FIGS. 8A and 8B are views showing the procedures of manufacturing anonvolatile memory cell as a semiconductor device according to the thirdembodiment. The procedures of manufacturing the memory cell shown inFIG. 3 will be described with reference to FIGS. 8A and 8B. FIGS. 8A and8B are sectional views in the word line direction (channel widthdirection).

First, as shown in FIG. 8A, a desired impurity is doped in a siliconsubstrate 101. A 7-nm thick tunnel insulating film 102 is formed on thesurface of the silicon substrate 101 by thermal oxidation. After that, aphosphorus-doped polysilicon layer 103 having a thickness of 150 nm isdeposited by low-pressure CVD as a prospective floating gate electrode.The phosphorus-doped polysilicon layer 103 and tunnel insulating film102 are sequentially etched by RIE using a resist mask (not shown). Inaddition, the exposed region of the silicon substrate 101 is etched toform a 150-nm deep element isolation trench.

A 400-nm thick silicon oxide film 107 a for element isolation isdeposited on the entire surface by plasma CVD to completely fill theelement isolation trench. After that, the surface is planarized by CMP.The exposed surface of the silicon oxide film 107 a is removed byetching using a diluted hydrofluoric acid solution to expose the sidesurface of the floating gate electrode 103 by 70 nm.

As shown in FIG. 8B, oxidation is executed in an oxygen atmosphere(oxidation atmosphere) containing 10% oxygen radicals at 800° C. for 1hr to form an 8-nm thick interelectrode insulating film 109 a made of aradical oxide film. Accordingly, a floating gate electrode 103 a obtainsa wide lower portion and a narrow upper portion. The opening widthbetween the interelectrode insulating films 109 a formed on the sidesurfaces of the adjacent floating gate electrodes 103 is larger than awidth obtained by subtracting a value twice the thickness of theinterelectrode insulating film 109 a from the minimum interval betweenthe floating gate electrodes 103.

Then, the memory cell structure as shown in FIG. 3 is completed by usingthe method shown in FIGS. 6A and 6B.

As shown in FIG. 8C, after the radial oxide film 109 a is formed, a CVDoxide film 109 b may be deposited to form an interelectrode insulatingfilm having a two-layered structure. As long as the lowermost layer is aradical oxide film, an insulating film of any type can be formed on it.A multilayered structure may be formed.

When the interelectrode insulating film 109 a or part of it is formed byradical oxidation, as in this embodiment, the interelectrode insulatingfilm 109 a can be formed at a relatively low temperature. Hence, thermaldamage of the tunnel oxide film decreases, and the degradation incharacteristics of the tunnel oxide film can be suppressed. In addition,the surface properties of the exposed element isolation oxide film canbe modified by radical oxidation. For this reason, the leakage currentbetween the adjacent floating gate electrodes 103 a can be reduced, andthe reliability of the memory cells can be increased.

Assume that an interelectrode insulating film having a three-layeredstructure including the silicon oxide film 109 a, silicon nitride film109 c, and silicon oxide film 109 d is formed, as shown in FIG. 8D. Whenthe silicon nitride layer 109 c is formed by thermal nitriding in a gasatmosphere containing nitrogen such as ammonia or nitrogen monoxide, thethickness of the nitride film can be decreased to about 1 nm. For thisreason, the buried portion of the control gate electrode can be wider.The same effect as described above can be obtained even when the siliconnitride film 109 c is formed by radical nitriding.

Assume that the silicon nitride film 109 c shown in FIG. 8D is formed byCVD using hexachloro-disilane and ammonia as source gases. In this case,since the electron trap density of the silicon nitride film 109 c ishigh, the nitride film thickness can be reduced to about 1 nm, whilesuppressing the degradation of data retention characteristics. For thisreason, the buried portion of the control gate electrode can be wider.Any other source gas combination can be applied if the electron trapdensity of a film can be increased by the film formation method.

The silicon nitride film 109 c shown in FIG. 8D may be changed to analumina film. The work function difference between an alumina film and asilicon oxide film is larger than that between an alumina film and asilicon nitride film. For this reason, the thickness of, e.g., analumina film can be reduced to about 1 nm, while suppressing thedegradation of data retention characteristics, and the buried portion ofthe control gate electrode can be wider. Any other insulating filmmaterial can be used as long as the work function is larger than that ofa silicon nitride film.

According to the embodiments of the present invention, the parasiticcapacitance between floating gate electrodes adjacent in the bit linedirection can be reduced while suppressing an increase in operatingvoltage by ensuring the coupling ratio of the memory cells. Accordingly,memory cell operation errors can be avoided. In addition, memory celloperation errors caused by depletion in the buried portion of thecontrol gate electrode can be avoided.

Memory cell operation errors caused by depletion in the control gateelectrode can be avoided while suppressing degradation incharacteristics of the tunnel insulating film. Since the surfaceproperties of the element isolation insulating film can be modified,leakage failures between opposing floating gate electrodes can beavoided. Since the interelectrode insulating film can be made thin,memory cell operation errors caused by depletion in the control gateelectrode can be avoided.

According to the embodiments of the present invention, a semiconductordevice which avoids a memory cell operation error caused by interferencebetween adjacent memory cells and a method of manufacturing the same canbe provided.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly; various modifications may be madewithout departing froth the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A method of manufacturing a semiconductor device, comprising: forminga plurality of floating gate electrodes on a tunnel insulating filmformed on a semiconductor substrate; filling a gap between side surfacesof the floating gate electrodes opposing each other with an elementisolation insulating film; removing part of the element isolationinsulating film to partially expose the side surfaces of the floatinggate electrodes; reducing a width, in a channel width direction, of anupper portion of each of the floating gate electrodes by etching part ofthe upper portion of each of the floating gate electrodes from theexposed side surfaces, thereby the upper portion being narrower than alower portion of the floating gate electrode, in the channel widthdirection; forming an interelectrode insulating film on the floatinggate electrodes, part of the interelectrode insulating film being formedon side surfaces of the upper portions of the floating gate electrodes;and forming a control gate electrode on the floating gate electrodes,the control gate electrode being partially buried between the floatinggate electrodes opposing each other.
 2. A method according to claim 1,wherein at least a part of a sidewall of the lower portion of each ofthe floating gate electrodes is in direct contact with a left or rightsidewall of the element isolation insulating film.
 3. A method accordingto claim 1, wherein a width, in the channel width direction, of theupper portion of the floating gate electrode is smaller than a width, inthe channel width direction, of a portion of the floating gate electrodewhere the floating gate electrode is in contact with the tunnelinsulating film.
 4. A method according to claim 1, wherein the etchingpart of the upper portion of each of the floating gate electrodes isperformed by using alkali solution.
 5. A method according to claim 1,wherein a width of an upper portion of the buried portion of the controlgate electrode is larger than a width obtained by subtracting a valuetwice a thickness of the interelectrode insulating film from a minimuminterval between the floating gate electrodes opposing each other.
 6. Amethod according to claim 1, wherein an oxide film as the interelectrodeinsulating film is formed in an oxidation atmosphere containing oxygenradicals.
 7. A method according to claim 1, wherein a nitride film asthe interelectrode insulating film is formed by one of thermal nitridingand radical nitriding.
 8. A method according to claim 1, wherein theinterelectrode insulating film is formed by sequentially forming a firstsilicon oxide film, a silicon nitride film, and a second silicon oxidefilm.
 9. A method according to claim 1, wherein the silicon nitride filmis formed by using hexachloro-disilane and a nitrogen containing gas assource gases.
 10. A method according to claim 1, wherein theinterelectrode insulating film is formed by sequentially forming a firstsilicon oxide film, an insulating film whose work function is largerthan that of a silicon nitride film, and a second silicon oxide film.11. A method according to claim 1, wherein: the lower portion of theplurality of floating gate electrodes are formed at a first intervalalong the channel width direction, and the upper portion of theplurality of floating gate electrodes are formed at a second intervalalong the channel width direction, the second interval being greaterthan the first interval.
 12. A method of manufacturing a semiconductordevice, comprising: forming a plurality of floating gate electrodes on atunnel insulating film formed on a semiconductor substrate; filling agap between side surfaces of the floating gate electrodes opposing eachother with an element isolation insulating film; removing part of theelement isolation insulating film to partially expose the side surfacesof the floating gate electrodes; oxidizing a surface of an upper portionof the floating gate electrode in an atmosphere containing oxygenradicals, thereby forming an oxide film; forming an interelectrodeinsulating film on the floating gate electrodes with the oxide filminterposed there between; and forming a control gate electrode on thefloating gate electrodes, the control gate electrode being partiallyburied between the floating gate electrodes opposing each other, whereinpart of the oxide film is located between an unoxidized portion of thefloating gate electrode and the element isolation insulating film.
 13. Amethod according to claim 12, wherein at least a part of a sidewall of alower portion of each of the floating gate electrodes is in directcontact with a left or right sidewall of the element isolationinsulating film.
 14. A method according to claim 12, wherein a width, inthe channel width direction, of the upper portion of the floating gateelectrode is smaller than a width, in the channel width direction, of aportion of the floating gate electrode where the floating gate electrodeis in contact with the tunnel insulating film.
 15. A method according toclaim 12, wherein a lower layer of the interelectrode insulating film isformed by the oxide film.